discord: add initial slash command integration.

Add initial support for the `/count <tags>` and `/posts <tags>` slash commands.

Slash commands are basically like webhooks; we register a command, and
when anybody types that command in Discord, Discord sends us a HTTP
request that we respond to.

* https://discord.com/developers/docs/interactions/slash-commands
* https://support.discord.com/hc/en-us/articles/1500000368501-Slash-Commands-FAQ

app/logical/discord_api_client.rb

@@ -0,0 +1,51 @@
+class DiscordApiClient
+  extend Memoist
+
+  BASE_URL = "https://discord.com/api/v8"
+
+  attr_reader :application_id, :guild_id, :bot_token, :http
+
+  def initialize(application_id: Danbooru.config.discord_application_client_id, guild_id: Danbooru.config.discord_guild_id, bot_token: Danbooru.config.discord_bot_token, http: Danbooru::Http.new)
+    @application_id = application_id
+    @guild_id = guild_id
+    @bot_token = bot_token
+    @http = http
+  end
+
+  def register_slash_command(name:, description:, options: [])
+    json = {
+      name: name,
+      description: description,
+      options: options
+    }
+
+    post("/applications/#{application_id}/guilds/#{guild_id}/commands", json)
+  end
+
+  def get_channel(channel_id)
+    get("/channels/#{channel_id}")
+  end
+
+  def me
+    get("/users/@me")
+  end
+
+  def get(url)
+    client.get("#{BASE_URL}/#{url}").parse
+  end
+
+  def post(url, data)
+    client.post("#{BASE_URL}/#{url}", json: data).parse
+  end
+
+  def client
+    headers = {
+      "User-Agent": "#{Danbooru.config.canonical_app_name} (#{Danbooru.config.source_code_url}, 1.0)",
+      "Authorization": "Bot #{bot_token}"
+    }
+
+    http.headers(headers)
+  end
+
+  memoize :client
+end